1. Field of the Invention
The invention relates to a process for the production of 6-methylnicotinic acid by oxidation of 2-methyl-5-ethylpyridine at an elevated temperature.
2. Prior Art
6-methylnicotinic acid ester has been produced by the oxidation of 2-methyl-5-ethylpyridine. Either potassium permanganate or nitric acid is used as an oxidation agent. The potassium permanganate process [J. Org. Chem. 24 (1959), 1190] has the disadvantage that large quantities of water and potassium permanganate and long reaction times are needed. Thus. 10.9 kg of potassium permanganate and 364 liters of water were needed per kg of end product. The reaction time was about 5 days and the yield was around 69 percent. According to C. A., Vol. 45 (1951), 9055, 2-methyl-5-ethylpyridine was converted with nitric acid in the presence of ammonium vanadate into 6-methylnicotinic acid, the latter being isolated as the copper salt. The reaction time was 3 days, and the yield was 51 percent. In U.S. Pat. No. 2,991,285, the preceding method was modified and the 6-methylnicotinic acid was isolated using ion exchange. In this case, the yield amounted to 46 percent. SeO.sub.2 with introduction of air has also already been described as the catalyst for the nitric acid oxidation (see U.S. Pat. No. 2,749,350). The product was isolated as the copper salt in a yield of 68 percent. After decomposition of the copper salt, the yield of 6-methylnicotinic acid was about 47 percent. The nitric acid oxidation has also already been carried out under pressure and at a temperature of from 130.degree. to 180.degree. C. (see U.S. Pat. No. 2,993,904). In that case, only extremely low yields of 6-methyl nicotinic acid were achieved.